1) Field of the Invention
This invention relates to a mobile communication terminal, and more particularly to a mobile communication terminal which adopts the CDMA (Code Division Multiple Access) communication system and uses two different systems of pilot signals to achieve improvement of the channel estimation precision.
2) Description of the Related Art
In a mobile communication system which adopts the CDMA communication system, taking it into consideration that the system capacity is increasing, it is essentially necessary to suppress the increase of the total transmission level from a radio base station. Therefore, synchronous detection which is superior in reception characteristic is adopted, and a radio base station transmits a pilot signal (known signal).
The pilot signal transmitted from the radio base station is received by a mobile communication terminal. The mobile communication terminal estimates (channel estimation) the phase rotation amount by which the received signal is to be corrected with the pilot signal and performs phase correction of the received signal to realize synchronous detection. As the pilot signal used here, usually an individual communication channel allocated to each mobile communication terminal and an interpolation pilot signal spread with the same code are available.
By the way, in recent years, a new technique is examined. According to the technique, a radio base station transmits, in addition to such an interpolation pilot signal as described above, an extrapolation pilot signal in which a fixed data pattern (for example, of all “0s”) is spread with a code different from that of a communication channel (but having the same frequency) using a channel common to different mobile communication terminals whereas a mobile communication terminal of the reception side uses the extrapolation pilot signal to estimate such a phase rotation amount as described above.
It is known that the estimation precision of the phase rotation amount is higher where a mobile communication terminal of the reception side uses an extrapolation pilot signal, which is transmitted successively, than where the mobile communication terminal uses an interpolation pilot signal, which is periodically embedded in a data slot. Thus, it is a possible idea to use only the extrapolation pilot signal without using the interpolation pilot signal.
However, in a mobile communication system which adopts the CDMA communication system, it is necessary to take a near-far problem in to consideration. In particular, TPC control (Transmission Power Control) is performed for a forward-link communication channel from a radio base station to a mobile communication terminal, and as the distance of the mobile communication terminal from the radio base station increases, the transmission power increases.
Accordingly, while an interpolation pilot signal interpolated in each individual communication channel is transmitted, by the TPC control, with higher transmission power as the distance of the mobile communication terminal from the radio base station increases, an extrapolation pilot signal is used commonly for different mobile communications and cannot be TPC-controlled for a particular one of the mobile communication terminals. In this manner, both of the interpolation and extrapolation pilot signals have merits and demerits, and one of them which should be adopted cannot be selected simply.